The semiconductor chip fabrication industry continues to strive for reductions in costs. One of the major strategies to reduce the production cost per chip is to migrate towards the use of larger diameter semiconductor wafers. Current semiconductor foundries primarily use 200 mm (8 inch) and 300 mm (12 inch) silicon wafers. By migrating to use of 450 mm wafers, the number of dies (of the same size) produced from each wafer will increase approximately in proportion to the growth in the area of the wafer. Thus, a 450 mm wafer can yield 2.25 times as many chips as a 300 mm wafer.
Processing larger wafers introduces mechanical challenges. One of the methods of providing a reliable process with a high yield is strict control over processing conditions. Because a 450 mm wafer has a larger diameter and surface area, it is more difficult to attain and maintain a uniform environment throughout the wafer while processing. For example, several processing steps are performed at specific temperatures. If heat or cooling is applied at discrete locations on the wafer, hot spots or cold spots may occur on the wafer. Additionally, secondary sources of heating and cooling (e.g., radiative heat transfer to or from the chamber walls) may affect the wafer unevenly. If the wafer temperature is not uniform throughout the wafer, then local variations may occur in various processing steps, causing within die variations and within wafer (between die) variations, such as line width variations.